CUDA is nVidia exclusive; it cannot and will not run on the CPU except as fallback if the GPU isn't working correctly.

If you turned off CUDA in your preferences, make sure that you also turned off "If no work for selected applications is available, accept work from other applications?" Leaving this option on, you are telling SETI that if there's no work for the programs you do accept, to go ahead and download work from other programs even if you haven't selected them.